Pounds, Kilograms and Gallons: Unit Mix-Ups That Change Your Answer

Short answer: one pound is exactly 0.45359237 kilograms, one US gallon is 3.785411784 litres, and one UK (imperial) gallon is 4.54609 litres. The shortcut of dividing pounds by 2.2 is close, but dividing by 2 is off by about 10%, and treating a UK gallon as a US gallon is off by about 20%. Most unit errors come from using the wrong version of a unit, not from the arithmetic.

What is the exact factor between pounds and kilograms?

The pound is defined as exactly 0.45359237 kg, and Mathify's weight converter uses that factor. Going the other way, 1 kg is 2.2046226218... lb.

We worked through a 180 lb body weight three ways:

• Exact: 180 x 0.45359237 = 81.6466266 kg.
• Dividing by 2.2: 81.82 kg, about 0.17 kg too high.
• Dividing by 2: 90 kg, about 8.35 kg too high.

And in the other direction, 75 kg is 165.3467 lb exactly, while multiplying by 2.2 gives 165, a shortfall of about 0.35 lb. Dividing by 2.2 is fine for a quick estimate, but not for a dose, a shipping weight or a body mass index. For the BMI case, see the BMI calculator. The earlier article on exact conversion factors lists more factors.

Are all stones, tons and tonnes the same?

No, and this is a classic mix-up. In Mathify's converter:

• One stone is 14 pounds (6.35029318 kg).
• One US short ton is 2,000 pounds, which is 907.18474 kg.
• One metric tonne is exactly 1,000 kg.

A "ton" can mean the short ton or the metric tonne depending on context and country, and the difference is about 9.3%. If a figure says "tons", check which one. The stone converter handles the UK body weight format.

Is a US gallon the same as a UK gallon?

No. A US gallon is 3.785411784 L. A UK (imperial) gallon is 4.54609 L. The ratio is 1.2009, so a UK gallon is about 20% bigger.

• 10 UK gallons = 12.0095 US gallons.
• 1 US gallon = 0.8327 UK gallons.

Mathify's volume converter lists both under separate names. Cup, pint and fluid ounce sizes in recipes also differ by country. The US versions in the converter are 1 US cup = 236.5882365 mL, 1 US pint = 473.176473 mL and 1 US fluid ounce = 29.5735295625 mL.

How do miles per gallon change between the UK and US?

Because the gallons differ, the same car gets different mpg figures:

• A car doing 30 miles per US gallon does 36.03 miles per UK gallon.
• A car quoted at 40 miles per UK gallon does 33.31 miles per US gallon.

A comparison across countries is only valid after converting. In litres per 100 km, 30 US mpg is 7.84 L/100 km, while 30 UK mpg is 9.42 L/100 km. We used 1.609344 km per mile for this. The fuel economy calculator works out consumption from your own distances and fuel, and the fuel economy guide covers trip costs.

Why can you not just double Celsius and add 30?

Temperature is different from the other units here because the scales start at different zero points. The conversion is F = C x 9/5 + 32, not a single multiplier. The popular shortcut of doubling and adding 30 drifts as the numbers grow. Computed with Mathify's temperature converter logic:

• 0 C is 32 F. The shortcut gives 30.
• 20 C is 68 F. The shortcut gives 70.
• 37 C is 98.6 F. The shortcut gives 104, which is 5.4 degrees too high.
• 100 C is 212 F. The shortcut gives 230.
• -40 C is -40 F, the point where the scales meet. The shortcut gives -50.

The shortcut is tolerable for weather between roughly 0 and 25 C and unsafe for cooking, lab work or anything with a threshold.

What about speed and distance?

The mile is exactly 1.609344 km, so 1 mph is 1.609344 km/h. At 70 mph that is 112.65 km/h. The rounded factor of 1.6 gives 112, which is fine for a road sign and not for a precise pace calculation. In the other direction, 100 km/h is about 62.14 mph. See the speed converter for knots and metres per second.

How do you avoid unit errors?

1. Write the unit next to every number. A bare 180 could be pounds or kilograms.
2. Name the variant. US or UK gallon, short ton or metric tonne.
3. Convert once, at the start, then keep one system throughout. Mixing systems inside one calculation is where errors multiply.
4. Sanity check the direction. A kilogram is bigger than a pound, so a weight in kg must be a smaller number than the same weight in lb.
5. Use exact factors. Keep rounding for the final display. See the length converter for inches, feet and miles.

Common mistakes

• Using 2 or 2.2 for kg to lb when the exact value is 2.2046.
• Dividing when you should multiply. The direction check above catches this.
• Assuming "gallon", "pint", "ton" or "cup" mean the same everywhere.
• Comparing mpg figures from different countries without converting.
• Rounding a factor early and then multiplying a large number by it.
• Converting units inside a formula that already expects a particular unit, such as BMI, which expects kilograms and metres.

Key takeaways

• 1 lb = 0.45359237 kg exactly. 1 kg = 2.2046226 lb.
• 1 US gallon = 3.785411784 L. 1 UK gallon = 4.54609 L. They differ by about 20%.
• A short ton is 907.18474 kg, a metric tonne is 1,000 kg.
• 30 US mpg is about 36.03 UK mpg.
• Always state the unit and the variant.

This article is about arithmetic and is not a substitute for guidance on medication or dosing.

FAQ

How many kilograms are in a pound?

0.45359237 kilograms, an exact figure by definition. To convert pounds to kilograms, multiply by 0.45359237.

Is 2.2 accurate enough for converting kg to pounds?

For a rough estimate, yes. 75 kg becomes 165 instead of 165.35 lb, an error of about 0.2%. Use the exact factor for anything that feeds another formula.

How many litres are in a US gallon?

3.785411784 litres. In a UK gallon there are 4.54609 litres.

Why is my car's mpg different on a UK website?

UK figures are usually per imperial gallon, which is larger. Convert 30 US mpg and you get about 36.03 UK mpg, which is why the same car looks more efficient.

Is a US ton the same as a metric ton?

No. A US short ton is 2,000 pounds, which is 907.18474 kg, and a metric tonne is 1,000 kg.